This invention is concerned generally with probes, and more specifically with probes for detecting and replicating high speed electronic signals with minimum disturbance of signal and maximum fidelity of replication, commonly used with devices for analyzing the detected signals, including, for example, oscilloscopes.
The usefulness of a probe depends upon the range of frequencies for which the response is true to the detected signal, the accuracy of replication, and the extent to which the probe detects the signal without detrimentally affecting the operation of the system or circuit being probed. If the input resistance of the combined probe and end-use device is the same order of magnitude as that of the circuit or system being probed, it may cause errors in the replication of the signal or a change in the operation of the circuit or system resulting in erroneous output or circuit malfunction. High probe tip capacitance will also cause circuit loading problems at higher frequencies. Designing the probe to have low capacitance and an input impedance which is very high relative to the impedance of the circuit being probed at the point of probing has been the common protection against these errors. This high impedance caused very little current to flow through the probe, allowing the circuit to operate relatively undisturbed.
The frequency response of a probe is dependent upon the capacitance of the probe in parallel with the source resistance of the circuit under test. The capacitive reactance varies as a function of frequency causing the impedance of the probe to vary with frequency. This has limited the effective bandwidth of prior art available probes, because the impedance of the probes falls at high frequencies. Minimizing the capacitance of the probe tip has been one solution for increasing the useful bandwidth of the probe. However, the probe tip capacitance has been proportional to the probe cable length, making tip capacitance difficult to erase. Compensating for the capacitance by using active electronics at the probe tip has been a second alternative, which has been used for extending the effective bandwidth of the probe tip. This generally has caused the probe tip to be bulky and easily damaged.
Typical probes available in the prior art included high resistance probes which minimized resistive loading and had high input impedance at dc, but the impedance fell off rapidly with increasing frequency due to high input capacitance. High impedance cable was used with these probes to minimize capacitance, but this cable was very lossy at high frequencies, limiting bandwidth. These probes also required the measuring instrument to have a high impedance.
Also available were passive resistive-divider probes which had the lowest input capacitances available in a probe and therefore had a very broad bandwidth. However, the low input impedance could cause problems with resistive loading which could force the circuit under test into saturation, nonlinear operation, or to stop operating completely.
A third type of probes were active field effect transistor probes which had active electronics at the probe tip to compensate for loading problems due to low input impedance. These probes had a higher input impedance than the resistive divider probes and a lower capacitance than the high impedance probes, but were limited in bandwidth by the available field effect transistors and were bulky and easily damaged.
In other fields, a concept called pole-zero cancellation has been known. One application in which the concept is used is a system for measuring heart rate disclosed in U.S. Pat. No. 4,260,951 of Lanny L. Lewyn. In that system, pole-zero cancellation was used to cancel the long differentiation time constant so as to remove undesired shaping of the heart pressure wave caused by the second order feedback loop. This allowed the waveform to be refined so that it could enable greater accuracy in measuring the heart rate.
More recently, wide bandwidth probes with pole/zero cancellation have been utilized in probe tips. In U.S. Pat. No. 4,743,839 of Ken Rush, a pair of tip components (the xe2x80x9ctip RC circuitxe2x80x9d) and a pair of feedback components (the xe2x80x9cfeedback RC circuitxe2x80x9d) are utilized. FIG. 1 shows a circuit diagram of the prior art probe circuitry of U.S. Pat. No. 4,743,839, the teachings of which are included herein by reference. Values for the components are chosen so that a zero created by the tip RC circuit 101 is at the same frequency as the pole created by the feedback circuit 102. The result of the probe circuitry is a constant gain over all frequencies. In addition, the terminating resistor 103, Rterm, is matched to the cable characteristic impedance, Z0, thus terminating the cable impedance to provide a constant, or flat, gain at all frequencies regardless of the cable length of the probe assembly. The feed back loop around the op-amp creates a virtual ground at the inputs, such that Rterm 103 is terminated into Vterm104.
The invention is generally to be used in probing devices of an electrical nature, with a preferred embodiment being used as a probe for a logic analyzer or an oscilloscope. The invention uses an application of the concept of pole-zero cancellation to improve the frequency response of the probe, and the concept of active termination of the coaxial cable in its characteristic impedance to provide a constant gain for the probe cable, independent of the length of the probe cable.
The operation of a probe tip in an embodiment of the invention utilized pole-zero cancellation. An resistor/capacitor (xe2x80x9cRCxe2x80x9d) circuit at the tip of the probe creates a zero at the same frequency that a feedback RC creates a pole. The resultant gain of the probe tip from the two RC circuits is a constant across frequency. Further, the cable impedance, Z0, is terminated into Rterm in a probe tip in accordance with the invention.
Additionally, a second pole and zero results from an embodiment of the invention. The additional pole is created by Rtap and Ctot. Ctot is the sum of the Ctip and the capacitance of the trace between Rtap and the tip RC, which is Ctap. By isolating Ctip behind Rtap, this pole contributes to reducing the load a target sees. The electrical length and impedance of the trace Ttap is variable, but the variations are accountable when calculating the pole crated at the tip.